9:00 AM - 9:15 AM
[SCG49-07] Experimental approach using ice polycrystals to understand the effect of dislocations on anelasticity
★Invited Papers
Keywords:anelasticity, dislocation, ice
In order to interpret the Earth's seismic structures, it is important to understand rock anelasticity caused by defects such as grain boundaries and dislocations. So far, anelasticity caused by grain boundary sliding has been intensively studied, and its effects on seismic wave velocity and attenuation can be predicted using experimentally derived models (e.g., Jackson & Faul, 2010; Yamauchi & Takei, 2016). In contrast, the effect of dislocations on anelasticity is poorly understood. Portions of the upper mantle where strong seismic anisotropy is observed are generally assumed to deform in a dislocation creep regime. In such areas, dislocations may strongly affect anelastic response. Although previous experimental studies showed that dislocations can enhance anelastic relaxation of rock (Gueguen et al., 1989; Farla et al., 2012), its quantitative assessment is still limited. The purpose of this study is to expand understanding of the effects of dislocation and deformation on anelasticity. If we were able to quantify the effects of dislocation density and/or strain-induced fabrics such as CPO on attenuation, we could use seismic methods to identify locations of active and relict deformation in the Earth.
Previous studies (Gueguen et al., 1989; Farla et al., 2012; Sasaki et al. 2019) performed two-stage deformation tests to investigate the effect of dislocations on anelasticity. In the first stage, dislocations were introduced in a sample by a creep test with a high stress in the dislocation creep regime. In the second stage, in a separate apparatus, attenuation of the pre-deformed sample was measured by the forced oscillation test with a low-amplitude cyclic stress in addition to a low offset stress. They observed that attenuation of the pre-deformed sample increased by the effect of dislocations. However, at the highest temperatures and under low offset stress dislocations annealed, and hence, the steady-state dislocation density could not be maintained during the second stage. This problem makes it difficult to quantify the effect of dislocations on anelasticity.
In this study, we aim to address this problem by using ice polycrystals as a rock analogue. Ice has an advantage that dislocation creep can be achieved relatively easily in the experimental condition with a moderate stress (Goldsby & Kohlstedt, 2001; McCarthy & Cooper, 2016). In this case, the pre-deformation up to 20% will take place in the cryogenic triaxial apparatus at U. Penn followed by attenuation measurements in the newly fabricated apparatus at Lamont. Although this study also performs two-stage deformation tests like the previous studies, we can apply an offset stress corresponding to the dislocation-creep regime during the forced oscillation test, which is easily obtainable for ice. Therefore, annealing of dislocations can be prevented, and hence, it is expected that dislocation density can be maintained in our experiments. We are now developing the new forced oscillation apparatus and also improving a method to observe dislocations in ice. These preliminary results will be shown in the presentation.
Previous studies (Gueguen et al., 1989; Farla et al., 2012; Sasaki et al. 2019) performed two-stage deformation tests to investigate the effect of dislocations on anelasticity. In the first stage, dislocations were introduced in a sample by a creep test with a high stress in the dislocation creep regime. In the second stage, in a separate apparatus, attenuation of the pre-deformed sample was measured by the forced oscillation test with a low-amplitude cyclic stress in addition to a low offset stress. They observed that attenuation of the pre-deformed sample increased by the effect of dislocations. However, at the highest temperatures and under low offset stress dislocations annealed, and hence, the steady-state dislocation density could not be maintained during the second stage. This problem makes it difficult to quantify the effect of dislocations on anelasticity.
In this study, we aim to address this problem by using ice polycrystals as a rock analogue. Ice has an advantage that dislocation creep can be achieved relatively easily in the experimental condition with a moderate stress (Goldsby & Kohlstedt, 2001; McCarthy & Cooper, 2016). In this case, the pre-deformation up to 20% will take place in the cryogenic triaxial apparatus at U. Penn followed by attenuation measurements in the newly fabricated apparatus at Lamont. Although this study also performs two-stage deformation tests like the previous studies, we can apply an offset stress corresponding to the dislocation-creep regime during the forced oscillation test, which is easily obtainable for ice. Therefore, annealing of dislocations can be prevented, and hence, it is expected that dislocation density can be maintained in our experiments. We are now developing the new forced oscillation apparatus and also improving a method to observe dislocations in ice. These preliminary results will be shown in the presentation.